Balloon first to block dead end (7)
I believe the answer is:
distend
'balloon' is the definition.
(I've seen this before)
'first to block dead end' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'block' means one lot of letters goes inside another (upper walls off glass inside formal).
'dead' becomes 'd' (abbreviation in government records).
'd'+'end'='dend'
'ist' inserted within 'dend' is 'DISTEND'.
(Other definitions for distend that I've seen before include "Swell, expand" , "blow up" , "Fill out" , "Cause to swell" , "balloon" .)